Exhaust gas discharged from an internal combustion engine of, for example, an automobile contains toxic components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Hitherto, three-way catalysts have been used for removing such toxic components for detoxifying the exhaust gas.
Such three-way catalysts include a noble metal (e.g., Pt, Pd, or Rh) serving as a catalytically active component; a material such as alumina, ceria, zirconia, or oxygen storing capacity ceria-zirconia composite oxide, serving as a carrier; and a catalyst support made of a ceramic or metallic material and having a shape of honeycomb, plate, pellet, etc. Recently, the regulation of automobile exhaust gas has been more strict, and the prices of Pt and Rh, which are noble metals serving as a main catalytically active component of internal combustion engine, exhaust gas purifying catalysts have risen. Under such circumstances, efforts have been made on reduction of exhaust gas purifying catalyst production cost by use of relatively cheap Pd as a catalytically active component, and various means therefor have been proposed (see, for example, Patent Documents 1, 2, and 3). In order to obtain enhanced exhaust gas purifying performance of Pd, studies have been intensively conducted on the functions of metal oxides such as Al2O3, CeO2, and ZrO2, serving as a catalyst carrier. However, to cope with variation in price of Pd due to increased demand thereof and to meet the demand of cost reduction by automobile manufactures, there is demand for an exhaust gas purifying catalyst which ensures effective cleaning performance by use of Pd in a reduced amount.